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Introduction

Updated 6/6/14: Shortly after this article was published, I learned the conducted power limits for Channels 36 - 48 have been raised from the 17 dBm (50 mW) used in our tests to 30 dBm (1 W). This obviously means that ASUS' RT-AC66U and RT-N66U are no longer in violation of the maximim conducted power FCC rules for the channels measured.

I have been told privately more than once that some manufacturers routinely and purposely ship routers with transmit power exceeding FCC limits. NETGEAR decided to do something about the alleged practice, filing suit against ASUS last year.

But does ASUS really ship routers with spec-busting transmit power? And does anyone else? I decided to do a little testing.

To keep things simple, I measured beacon pulse power because it's a nice clean signal repeated at a set interval. For measurement, Rohde & Schwarz (R&S) graciously loaned me an NRP-Z11 Power Sensor. This is R&S' top-of-line three-path diode power sensor with a USB output that connects to a base unit, or a Windows, MacOS or Linux computer running R&S' Power Viewer Plus software.

Rohde & Schwarz NRP-Z11 Power Sensor

I have a quiet Wi-Fi environment and was measuring conducted, not radiated, power. So I just set everything up on my office bench as shown in the photo below. Note the 20 dB fixed attenuator added to the R&S sensor's cable to prevent overload. The Power Viewer Plus conveniently allows you to enter an offset so that you can read corrected measurements directly off the screen.

Transmit power test setup

In the U.S., maximum power delivered to the antenna (conducted power) is limited as shown in Table 1. Note that this is the total power of all transmit chains. For devices with more than one transmit chain, per chain power limits are adjusted as shown in the table.

Band

Limit (dBm)

1x1

2x2

3x3

4x4

2.4 GHz

30

27

25.2

24

5 GHz U-NII  Band-I
(Ch. 36 - 48)

17

14

12.2

11

5 GHz Band-IV
(Ch. 149 - 165)

30

27

25.2

24

Table 1: Maximum conducted transmit power limits

Table 1 is a simplified form of the actual rules, which got a lot more complicated when technologies like MIMO, beamforming and smart antennas came into play. In FCC testing, these limits are adjusted downward if array gain due to various MIMO techniques exceeds 6 dBi. Since I wasn't looking for minor variations from spec, I didn't bother making these adjustments. As it turned out, I didn't have to.

Results

I took detailed data on five 802.11ac routers and one 802.11n that are listed alphabetically in Table 2. I first updated each router to the most recent firmware posted and included the firmware revision in the table. For all the routers in the table, "Chain 0" is the left-most antenna facing the front of the router, with the others in order from left to right. For 2.4 GHz measurements, I set channel bandwidth to 20 MHz. For 5 GHz, I used the Auto or 20/40/80 MHz setting to ensure 802.11ac operation.

Measurements exceeding the FCC limits are in bolded red. All these routers were easy to measure because they had external RP-SMA antenna connectors. So all I had to do was unscrew the antennas and screw on the R&S sensor cable.

All the power values in Table 2 are shown in dBm. The total column is the sum of all active chain powers, calculated by converting each chain power to mW, summing the values, then converting back to dBm.

Router

Channel

Power (dBm)

Chain 0

Chain 1

Chain 2

Chain 3

Total

Limit

ASUS RT-AC68U

F/W: 3.0.0.4.374_5656

1

18

18.1

17.8

-

22.74

30

6

17.9

17.7

16.8

-

22.26

30

36

5.6

5.8

5.9

-

10.54

17

48

5.4

6.8

6.2

-

10.94

17

149

21.1

20.8

21

-

25.74

30

153

21.1

20.8

20.9

-

25.71

30

ASUS RT-AC66U

F/W: 3.0.0.4.374_5517

1

22.9

23.6

22.7

-

27.86

30

6

22.9

23.6

22.1

-

27.68

30

36

20.5

19.7

20

-

24.85

17

48

20.6

20

20.8

-

25.25

17

149

20.7

19.7

20

-

24.93

30

153

20l8

19.7

20.2

-

25.03

30

ASUS RT-N66U

F/W: 3.0.0.4.374_5517

1

21.8

21

21.4

-

26.18

30

6

22

21

21.4

-

26.43

30

36

21.4

21.5

20.8

-

26.02

17

48

21.5

21.9

21

-

26.25

17

149

20.2

21.2

20.7

-

25.45

30

153

20.1

21.1

20.7

-

25.42

30

Linksys EA6900

F/W: 1.1.7.160582

1

21

20.7

20.7

-

25.57

30

6

16.4

15.4

15.9

-

20.69

30

36

3.4

4

4.5

-

8.76

17

48

3.9

4.1

4.6

-

8.98

17

149

17.8

18.1

18.6

-

22.95

30

153

18.1

18.3

19.3

-

23.37

30

Linksys WRT1900AC

F/W: 1.1.7.160582

1

22.3

22.8

21.3

22

28.15

30

6

22

22.6

21.3

21.9

28

30

36

6.7

8.2

6.9

5.7

12.99

17

48

6.7

8.4

7.8

6.2

13.38

17

149

21.5

18.6

19.7

17.5

25.6

30

153

20.2

17.8

17.3

15.4

24.04

30

NETGEAR R7000

F/W: 1.1.7.160582

1

21.8

22.5

22.3

-

26.98

30

6

23.5

24

23.7

-

28.51

30

36

11.9

11.5

11.2

-

16.31

17

48

12.3

11.3

11.8

16.59

17

149

21.1

21.4

19.4

-

25.49

30

153

21.1

21.5

19.1

-

25.46

30

Table 2: Test results

The results show only two routers—ASUS' RT-AC66U and RT-N66U—exceeded the FCC limit. This happened only on the 5 GHz U-NII  Band-I channels (36 - 48) where the power limit is 17 dBm (50 mW). The data shows those channels' transmit power limit appeared to be treated the same as the Band-IV channels' (149 - 165), i.e. 30 dBm (1 W). The screenshot below shows R&S' Power Viewer Plus screen while measuring the RT-AC66U set to Channel 48.

ASUS RT-AC66U - Channel 48

It's interesting to note that ASUS' AC1900 class RT-AC68U did not exceed the FCC limits. In fact, its total transmit power for Channels 36 and 48 was next to lowest among the measured routers at just under 11 dBm.

ASUS Transmit Power Settings Revealed

While I had the power sensor connected, I checked how much ASUS' power setting range of "1mW" to "200mW" actually changed power. On the RT-AC66U, the default 80mW setting produced ~23 dBm from one of the 2.4 GHz radio's chains. Changing the setting to 1mW dropped the power to 11 dBm, but raising it to the full 200mW moved power up only 1 dBm.

Assuming the same power on all three chains, changing the transmit power setting from 80mW to 200mW would move total power from around 586 mW to 720 mW. Not quite the 2.5X change implied by ASUS' "mW" settings.